As for an image forming element in which a cobalt complex is used, the elements given in, for example, Japanese Patent Publication Open to Public Inspection (hereinafter referred to as JP OPI Publication) Nos. 50-139722/1975, 50-139723/1975 and 50-139724/1975 have been known. Further, the image forming elements, in which a cobalt complex and a phthalaldehyde are used, are described in the above-given patents and JP OPI Publication Nos. 52-101026/1977 and 57-63527/1982.
The elements described in the above-mentioned patents are excellent in such a point that an image can be formed in a simple and easy dry process and by the functions of phthalaldehyde and amine, upon heating the elements after exposing them to light.
However, a cobalt complex is short of photosensitivity in itself, so that it is limited only to slightly show a low photosensitivity. With an image forming element described in the above-mentioned patents, a sensitivity is therefore provided thereto by making use of such a photoreducing agent as a naphthoquinone derivative and such a photoxidizing agent as a naphthol derivative, as a photosensitive substance. However, not only it cannot be said that the resulting photosensitivity is satisfactorily high, but there has been such a defect that the subject element is inferior in preservability because a naphthoquinone derivative and a naphthol derivative are reduced or oxidized in preservation, or that a stain is produced by a photoreducing agent or a photoxidizing agent after completing an image formation.